Thermal dye sublimation transfer also called thermal dye diffusion transfer is a recording method in which a dye-donor element provided with a dye layer containing sublimating dyes having heat transferability is brought into contact with a receiver sheet and selectively, in accordance with a pattern information signal, heated with a thermal printing head provided with a plurality of juxtaposed heat-generating resistors, whereby dye from the selectively heated regions of the dye-donor element is transferred to the receiver sheet and forms a pattern thereon, the shape and density of which is in accordance with the pattern and intensity of heat applied to the dye-donor element.
A dye-donor element for use according to thermal dye sublimation transfer usually comprises a very thin support e.g. a polyester support, one side of which is covered with a dye layer, which contains the printing dyes. Usually an adhesive or subbing layer is provided between the support and the dye layer. Normally the opposite side is covered with a slipping layer that provides a lubricated surface against which the thermal printing head can pass without suffering abrasion. An adhesive layer may be provided between the support and the slipping layer.
The dye layer can be a monochrome dye layer or it may comprise sequential repeating areas of different colored dyes like e.g. of cyan, magenta, yellow and optionally black hue. When a dye-donor element containing three or more primary color dyes is used, a multicolor image can be obtained by sequentially performing the dye transfer process steps for each color.
Many of the dyes proposed for use in thermal dye sublimation transfer are not sufficient in performance because they yield inadequate transfer densities at reasonable coating coverages. Especially for transfer on transparant film materials as receiving element, the transfer densities obtained are too low.
For obtaining sufficient cyan densities on transparant film receiving elements the class of the so-called indoaniline dyes (as described e.g. in U.S. Pat. No. 4,829,047) and the class of cyan dyes as described in U.S. Pat. No. 5,026,677 are well suited.
A primary colored dye layer, for example, a magenta or cyan or yellow dye layer, may comprise only one primary colored dye (a magenta, cyan or yellow dye respectively) or may comprise a mixture of two or more primary colored dyes of the same hue (two magenta, two cyan or two yellow dyes respectively).
The so-called additive colors green, red and blue in the obtained prints are realised by printing sequentially the primary colors on each other; thus a green color on the print is obtained by printing sequentially cyan and yellow colored dyes, blue by printing sequentiallly cyan and magenta colored dyes. So in fact these shades consist of mixtures of dyes.
Dye images containing mixtures of dyes frequently show an increased fading rate due to a photochemical effect, known as catalytic fading of dye mixtures.
This phenomenon was investigated in textile dyeing by Rembold and Kramer (see Journal of the Society of Dyers and Colourists, vol. 94 (1978), pages 12-17) and by Asquith and Ingham (see Journal of the Society of Dyers and Colourists, vol.89 (1973), pages 81-85). Catalytic fading is related to the observation that the lightfastness of certain dyes applied to textiles alone is much better than when applied as mixtures. In most reported cases, the lightfastness of cyan, violet or red dyes deteriorates when a yellow dye is added.
Indoaniline dyes have a favourable effect on catalytic fading especially in dye mixtures for obtaining black colored images. However there are still strong catalytic fading effects in the green colored images composed of yellow and cyan dyes, with cyan indoaniline dyes as well as with other cyan azomethine dyes. Moreover, many indoaniline dyes have poor light stability.
Therefore, it is an object of the present invention to provide dye-donor elements, in particular cyan dye donor elements yielding transferred dye images of high density showing no or decreased catalytic fading effects in the green colored image areas.
It is another object of the present invention to provide cyan dye-donor elements, containing mixtures of cyan colored dyes, yielding transferred cyan dye images showing improved light stability.
Other objects will become apparent from the description given hereinafter.